1. Field of the Invention
The present invention relates to the field of lithium ion batteries. This invention also relates generally to improved electrodes for lithium ion batteries and methods for making same.
2. Related Art
Due to the exponential growth in global energy consumption, rapid depletion of fossil fuels, concomitant growth in greenhouse gas emissions, and the upward spike in the prices of crude-oil and gasoline, significant concerns and efforts have been focused on the development of clean and renewable energy sources and advanced energy storage technologies. Lithium-ion batteries are the most popular rechargeable energy storage devices in consumer electronics and main contenders for powering commercially viable electric vehicles in the near future.
Further development of high-performance rechargeable lithium-ion batteries (LIBs) is indispensable for the ever growing needs for electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). Remarkable research efforts have been devoted to improving the already incomparable performance of rechargeable LIBs which are ubiquitous in various fields since its successful commercialization about 20 years before. The most popular graphite-based anodes, with a theoretical specific capacity of 372 mAh/g, are commonly used in commercially available rechargeable LIBs along with several types of Li oxide based cathodes (less than 170 mAh/g specific capacity). However, current graphite and transition metal oxide based electrodes only provide moderate energy-storage capability and therefore it is difficult for them to meet the increasing demands for advanced energy storage. Hence, it is essential to design and synthesize new anode materials that can offer the promise of high-performance LIBs with elevated efficiency, superior storage capacity and gravimetric energy density, longer cycle life, easier state-of-charge control, lower cost, and safer operation.
Silicon-based electrodes for rechargeable LIBs have attracted considerable attention because they are able to vastly improve the specific capacity of batteries. As a naturally abundant element, silicon has the highest theoretical specific capacity among all exiting materials, which can reach 4200 mAh g−1 in the form of Li4.4Si. Furthermore, Si is also inexpensive, easy to handle, and has low discharge potential when used as an anode for rechargeable LIBs. These unique attributes endow Si as one of the most promising candidates to replace graphite as the anode for high performance rechargeable LIBs.
Unfortunately, silicon's potential in broad commercial applications has been hindered by severe capacity fading and loss of electrical contact caused by large volume change, structural crumbling, and even cracking during repeated charge and discharge cycling, especially at high current rates. Downsizing from conventional bulk silicon to various nanoscale morphologies and structures or dispersing these nanostructured Si into carbon matrices are among the most appealing approaches being pursued to overcome these issues and to improve the overall electrochemical performance of Si-based anodes in rechargeable LIBs.
For example, studies on electrodes made of Si bulk films and particles of the order of microns revealed severe capacity fade and short cycle life due to structural crumbling and/or cracking. On the other hand, electrodes made of Si thin-films, nano-wires, and nano-particles showed a marked improvement in the fracture performance. Clearly, the surface area per unit mass increases in inverse proportion to the particle size. Although decreasing the particle size improves the rate of lithiation/delithiation and fracture resistance, it also offers large surface area for electrolyte-reduction reactions resulting in the formation of solid-electrolyte-interphase (SEI) layer and the associated irreversible loss of Li. It has been shown that capacity loss to SEI formation on graphite electrodes was proportional to the surface area of the electrode; assuming the formation of a Li2CO3 film, a SEI thickness of 4.5 nm on carbon particles was calculated, consistent with the barrier thickness needed to prevent electron tunneling.
The SEI layer plays an important role in the safety, power capability, and cyclic life of Li-ion batteries. In one of the earliest works on the SEI, a model was proposed for a SEI formation mechanism in non-aqueous electrochemical systems such as Li-ion batteries that concluded that formation of a chemically and mechanically stable SEI layer is the key for improving the cycle life of batteries. For example, Chen et al. enhanced the electrochemical performance of Si electrodes by improving the properties of SEI layer (achieved by adding vinylene carbonate additive in their electrolyte).
Recently, it was shown that additives such as propylene carbonate, lithium difluoro-oxalatoborate, and fluoro-ethylene carbonate dramatically improve the cyclic efficiency of Si electrodes. Lee et al. found that SEI layer on Si electrode forms due to reduction of organic solvents and anions at the electrode surface during charging and discharging cycles of batteries; bulk of the formation occurs during the first cycle. Yu-Chan et al. characterized SEI layers formed on Si electrodes and found fluorinated C and Si species, besides the usual Li2CO3, alkyl Li carbonates (ROCO2Li), LiF, ROLi, and polyethylene oxides that are found on graphite electrodes. SEI formation on the negative electrode is an irreversible reaction that consumes cyclable Li-ions from the positive electrode leading to most of the capacity loss observed in the first lithiation/delithiation cycle of secondary lithium-ion batteries.
Besides capacity loss in the first cycle, continuous formation of this layer also increases resistance to Li-ion diffusion (i.e., internal impedance of a battery). In spite of the important role played by the SEI layer on the calendar and cycle life of secondary lithium-ion batteries made with Si anodes, there have not been many studies on understanding the mechanisms of initial formation of SEI on Si electrodes. Furthermore, there have been few attempts towards quantifying the first-cycle capacity loss due to SEI-layer formation on Si. Since the requirements for fracture tolerance (i.e., small particle size) would be in conflict with the need to minimize the first cycle irreversible loss (i.e., minimize the surface area per unit mass), measurement of Li loss per unit area due to irreversible reactions can serve as a useful design parameter in arriving at optimal micro/nano architectures for Si-based electrodes. Furthermore, quantifying the charge lost to SEI formation is essential for accurately arriving at the true state-of-charge of the silicon electrode during its initial lithiation. For in situ measurements of stress as well as the mechanical properties of a silicon electrode during its initial lithiation, it is essential to know the exact Li concentration in Si electrode to calculate its volumetric strain.
Graphene, a new class of two-dimensional, “aromatic,” monolayer of carbon atoms densely packed in a honeycomb crystal lattice, has attracted unmatched attention and has also triggered tremendous experimental activities for applications in next generation electronic and energy storage devices, owing to its exceptional properties including extraordinarily high electronic mobility, outstanding optical transparency, unique electronic structures, intriguing thermal conductivity, and amazing mechanical strength as well as ultrahigh surface area. Hence, graphene could be superior to other carbon materials as a conductive matrix to enhance electron transport and electrical contact with Si active materials in rechargeable LIBs and to effectively prevent the volume expansion/shrinkage and aggregation of Si phases during the Li charge/discharge processes. Furthermore, its large surface area can also facilitate the absorption of Li atoms on both sides of the graphene sheet or into its ubiquitous cavities. As a result, the merits of both carbon and Si phases can be extended to the largest extent based on their synergetic effects.
Recently, Chou et al. (S.-L. Chou, J.-Z. Wang, M. Choucair, H.-K. Liu, J. A. Stride, S.-X. Dou, Electrochemistry Communications 2010, 12, 303) blended commercially available nanosized Si particles and graphene to prepare eco-friendly and low cost LIB anodes, which exhibited enhanced cycling stability. In the meantime, several other groups also successfully prepared Si nanoparticles/graphene paper composite as anodes for rechargeable LIBs with high Li storage capability and cycling stability. (See J. K. Lee, K. B. Smith, C. M. Hayner, H. H. Kung, Chemical Communications 2010, 46, 2025: G. Wang, B. Wang, X. Wang, J. Park, S. Dou, H. Ahn, K. Kim, Journal of Materials Chemistry 2009, 19, 8378) The studies also indicated that graphene can be used to anchor electrochemically active transition metal oxides or metal nanoparticles as anode materials for rechargeable LIBs, and these batteries exhibit enhanced cycle life and improved reversible capacity. See, for example, US Published Patent Application 2011/0033746, filed Aug. 10, 2009. The use of Si nanoparticles, however, may not provide a simple way to optimize the ion transport in the anode, especially when the loading of Si is high. Furthermore, required is the use of inactive binders to hold the Si and Carbon components together, which serves to reduce the overall energy capacity.
Polyvinylidene difluoride (PVDF) has been the widely used binder materials for both cathode and anode electrode. Water soluble styrene-butadiene rubber (SBR) binder has also been tested for LiFePO4, LiCoO2 and Graphite electrodes. Other binders have also been tested. Other potential commercial binders in lithium ion battery electrodes other than PVDF are SBR type of binders for graphite anode application. However, the reported performance based on non-PVDF binders is lower than the performance for PVDF binder electrodes.